The proposed studies employ the fetal and newborn lamb as experimental models for the study of thyroid and pituitary-thyroid physiology in the fetal and neonatal periods. Included are studies of the maturation of hypothalamic-pituitary control of thyroid function in the fetus; studies of the mechanism(s) of fetal triiodothyronine deficiency; studies of maturation of iodothyronine deiodinating systems in the fetus and newborn; studies of the mechanism(s) of neonatal thyroidal hyperactivity; and studies of the role of thyroid hormones in neonatal thermogenesis.